This invention relates to the pickling of metals by passing the same through an acid bath, and in particular to the reclamation of unused acid from and concentration of spent pickle liquor produced by a metal pickling process.
In the production of metals, and in particular ferrous metals such as steel, metal oxide scales often form on the surface of the metals. To remove such scale, the metal is typically passed through an acid bath referred to as pickle liquor. Frequently, the pickle liquor used is hydrochloric or sulfuric acid. As the metal is treated, metallic salts are produced by the corrosive action of the acid bath. The corrosive action also occurs on the unoxidized or descaled portions of the metal being pickled, which is an undesirable consequence of the pickling process as the metal so removed is wasted. Sometimes an inhibitor such as thiourea is included in the bath which somewhat inhibits the corrosion of the unscaled metal while having little effect on the descaling action. The formation of the metallic salts uses and thus reduces the concentration of the pickle liquor acid, and as will be discussed later, also often undesirably enhances the corrosive effect on the descaled metal. Hence, the metallic salts must be removed from the pickle liquor, and fresh acid added thereto. Spent pickle liquor which is relatively high in metallic salts and low in acid is thus typically bled off from the pickle liquor bath. The unused acid in the spent liquor is, therefore, lost unless recovered. Disposal of the spent liquor containing unused acid is difficult both due to total volume of the spent pickle liquor and low PH caused by the acid. Federal and state regulations on dumping of low PH materials are becoming increasingly strict, thus this low PH material generated from the pickling process normally requires neutralization before placing in a landfill, deep welling or other disposal site.
A number of processes have been developed for converting the metallic salts of various ferrous metal pickling processes, wherein the pickling acid is hydrochloric acid, to iron oxides, often by combusting oxygen and fuel in the presence of the iron chlorides produces therein. However, these processes require substantially increased inputs of energy and often need careful monitoring of the operation. Hence these processes may require substantial additional energy and labor inputs as compared to the requirements of operating a pickling process without any recovery system.
In addition, spent pickle liquor metallic salt concentration processes are sometimes used in conjunction with pickling processes using hydrochloric acid which include a single stage evaporator and the absorption or condensation of the acid-water gaseous vapors or overheads released from the evaporation before returning of the overheads to the pickling bath. This process in general uses separate energy supplies in the form of heat to vaporize the acid-water mixture and to heat the pickling baths to their proper operational temperature. Thus, most spent pickle liquor concentration processes require substantially greater energy input than does a pickling process which does not concentrate the spent pickle liquor.
The marketability of some of the metallic salts is low and thus they are disposed of by standard methods of waste disposal, however, many of the salts such as iron chlorides have a relatively good commercial market. Normally salability improves as the salt concentration of the solution containing the metallic salts increases, and as the free or unused acid concentration therein decreases. Marketability is also significantly enhanced by a decrease in free or unused acid content of the concentrated metallic salts solution. Therefore, the unused acid removal and concentration of the remaining salt solution provided by the present invention, as will be discussed later, also improves the value and salability of the concentrated spent pickle liquor. In addition, the ferrous chlorides generated in the ferrous metal pickling processes using hydrochloric acid are not as marketable as ferric chlorides and it is thus desirable to convert the ferrous chloride to ferric chloride.